1. Field of the the Invention
The present invention relates to a membrane multi-layer structure which can be utilized as a non-volatile semiconductor memory and the like by providing a dielectric layer made of a ferroelectric on a semiconductor substrate, and a capacitive element, actuator element and filter element using the same.
2. Prior Art
Polarization of a ferroelectric layer is derived from a crystal of a substance, and when a ferroelectric layer of an oxide is utilized, a polarization property can be obtained by heating at several hundred degrees in the presence of oxygen to cause crystallization. For mounting this ferroelectric as a thin layer to obtain large polarization, an oxide single crystal layer having a uniform composition and having no defects is necessary.
However, in many cases, it is very difficult to obtain a single crystal layer, and usually, only a polycrystal is obtained. On the other hand, a polycrystal shows increase in leakage current due to presence of a defect of grain boundary and the like, and shows smaller polarization than that of a single crystal due to irregular orientation of crystal particles. Therefore, a layer not only oriented to the vertical direction to a base as standard but also oriented so as to have the same plane as the base plane, a so-called epitaxial layer, is preferable for obtaining large polarization.
Because of this, for allowing an epitaxial layer of a dielectric to grow, orientation of the surface of a single crystal semiconductor substrate such as a silicon substrate and the like is utilized. However, this silicon single crystal substrate is made of metal, and when exposed to an oxygen atmosphere at higher temperatures, the surface is easily oxidized to produce a silicon oxide layer (SiO2). Since the silicon oxide layer is amorphous and has no orientation, it is difficult to allow an epitaxial layer of a dielectric to grow on this. Further, that reaction and diffusion between a ferroelectric layer to be grown and a silicon single crystal substrate are weak is also important for epitaxial grow of a ferroelectric layer. For example, The non-patent literature entitled “Mat. Res. Soc. Sym. Pro. Vol. 341” (1944, P. 73) discloses examples such as oxides of rare earth elements such as yttrium-stabilized zirconia (hereinafter, referred to as YSZ), cerium oxide and the like, magnesium oxide, magnesia spinel, strontium titanate and the like as the substance which can be epitaxial-grown on a silicon ingle crystal substrate.
For preventing a reaction of a silicon single crystal substrate and a simple perovskite oxide of a ferroelectric and formation of SiO2 on the surface of a silicon single crystal substrate, it has been tried to form an intermediate layer made of yttria-stabilized zirconia (YSZ) formation of which epitaxial layer on a single crystal substrate had been reported, and magnesium oxide, magnesia spinel and the like, and to cause epitaxial growth of an oxide having a perovskite structure on the top of the intermediate layer. For example, regarding structures realizing the (001) epitaxial layer having a perovskite structure on the silicon (001) substrate, the non-patent literature entitled “J. Appl. Phys. Vol. 66” (1989,P. 5826) discloses an epitaxial structure of SrTiO3 (001)/MgAl2O4 (001)/Si (001), and the non-patent literature entitled “J. Crystal Growth Vol. 158” (1996,P. 84) discloses an epitaxial structure of PZT (001)/MgO (001)/Si (001).
Further, a zirconium oxide layer is also supposed as an intermediate layer since it can epitaxial-grow on a silicon substrate. The Non-patent literature entitled “Appl. phys. Lett. Vol. 67” (1995,p. 1387) discloses that when a substance having a perovskite structure is formed on a zirconium oxide layer, a perovskite structure (001) plane grows in epitaxial relation to a zirconium oxide (001) plane.
However, since polarization of a ferroelectric having a simple perovskite structure has directionality, it is necessary to orient a ferroelectric to a coincident direction of a polarization direction and a direction of motion. For example, a non-volatile memory (FeRAM) using a ferroelectric having spontaneous polarization in a capacitor portion is expected to application to IC cards and the like as a memory of the next generation. In capacitor portions such as a non-volatile memory (FeRAM) and the like, a tetragonal ferroelectric perovskite oxide is usually used, and residual polarization direction of a tetragonal ferroelectric resides on the (001) plane. When a perovskite oxide on a zirconium oxide layer is oriented to the (011) plane, direction of residual polarization becomes inclined, and a high dielectric constant cannot be obtained, inviting decrease in dielectric property. This problem is a problem common not only to a non-volatile memory (FeRAM) but also to a capacitive element, actuator element and filter element.
JPA1997-110592 discloses that a laminated thin layer provided as an intermediate layer between a perovskite layer and a zirconium oxide layer to which a rare earth oxide such as CeO2 and the like is added can give an epitaxial thin layer oriented to the (001) direction of an oxide having a simple perovskite structure. In this case, the lattice constant of zirconium oxide is 0.51 nm, the lattice constant of a typical simple perovskite oxide such as barium titanate and PZT and the like is about 0.39 nm, and the proportion of unconformity (here, represented by (1−0.39/0.51)×100%) is 23%. The plane interval on the (011) plane of a simple perovskite oxide is (2)1/2×0.39 nm≈0.55 nm, and the proportion of unconformity of crystal lattice is as small as −8%. It is supposed that by this, epitaxial growth is possible.
However, it is difficult to obtain a simple perovskite layer completely oriented to the (001) direction, and in many cases, orientation to the (011) direction remains and high dielectric constant cannot be obtained. Its crystalline property is also low, further, orientation is low, so polarizability cannot be increased. However, JPA1997-110592 describes that a barium titanate layer grows without causing in-plane rotation against a zirconium oxide layer, and three lattices of zirconium oxide (0.51 nm×3=1.53 nm) grow in conformity to four lattices of barium titanate (0.39 nm×4=1.56 nm). However, a grown layer including such large unconformity shows a tendency of concentration of stress on the interface and a tendency of peeling. Further, since conformity of lattices is not good, a crystalline property is also low. Moreover, JPA1997-110592 describes that a simple perovskite oxide grows without rotation at the (001) plane on a zirconium oxide layer, and it is difficult to obtain a layer of constitution in which conformed epitaxial growth is made by 45° in-plane rotation.